1. Field of the Invention
The present invention relates to the production of very low sulfur content gasoline. More particularly the invention relates to a process for the production of very low sulfur gasoline from naphtha stocks containing significant amounts of unsaturated compounds including olefins, diolefins and acetylenes.
2. Related Information
Governments worldwide are requiring lower and lower organic sulfur contents in motor gasolines because of the noxious sulfur oxides which are produced in their combustion. The United States is expected to lower its limits on total organic sulfur contents to less than 50 weight parts per million.
This requirement comes on the heels of government requirements for cleaner burning fuels containing more oxygenated compounds such as ethers or alcohols.
Octane, the index of how well a gasoline performs in an internal combustion, is always a much sought after characteristic. The basis for measuring octane is a comparison to 2,2,4 trimethyl pentane (isooctane) whose octane number is 100. Other hydrocarbons which exhibit a good octane number are aromatics and olefins. Of the aromatics, benzene, because of its known carcinogenic properties, is not desirable as a gasoline component.
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determines the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule, most of the “sweet” (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Some users require the sulfur of the final product to be below 50 wppm.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:RSH+H2→RH+H2S  (1)RCl+H2→RH+HCl  (2)2RN+4H2→2RH+2NH3  (3)ROOH+2H2→RH+2H2O  (4)
Typical operating conditions for the HDS reactions are:
Temperature, ° F.600-780Pressure, psig 600-3000H2 recycle rate, SCF/bbl1500-3000Fresh H2 makeup, SCF/bbl 700-1000After the hydrotreating is complete the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha. The loss of olefins by incidental hydrogenation has been considered detrimental due to the reduction of the octane rating of the naphtha and the reduction in the pool of olefins for other uses.
However, it has been found that H2S recombines with olefins in a naphtha to produce mercaptans. These recombinant mercaptans make it difficult to achieve the lower sulfur levels being required.
At one time it was thought necessary to “reform” heavy fluid cracked naphtha. The reforming process requires that sulfur and nitrogen contaminants be almost nonexistent, that is, less than 0.5 weight parts per million. In addition olefins caused coking of the reforming catalysts and thus the content was required to be less than one percent by volume. Generally only straight run naphthas had been fed to reformers contained little if any olefinic compounds and sulfur compounds which were easily removed. The traditional reformer pretreatment hydrotreaters were satisfactory for the earlier reformer feeds. However, the heavy fluid cracked naphthas required severe treating conditions to remove the sulfur, nitrogen and olefin contaminants to the desired levels. Pressures in excess of 1100 psig (hydrogen partial pressures of greater than 600 psia) and temperatures above 650° F. were necessary. In the end the high severity of treating and lowering of projected octane requirements made this process neither feasible nor necessary.